People say "I could care less" - all the time when they actually mean "I could not care any less."
To say, "I do not care" or "I do not care at all" may be the most correct, but it does not have the insulting tone that "I could not care less" has. The subtext is "I do not care about you."
Therefore, if you truly do not care, use, "I don't care." If you actually care and want to say you do not, use "I could not care less!"